Modern Warfare 2 players over the past two weeks. The cause for its insatiable ban lust? Valve president Gabe Newell explained in an email to those affected, "the problem was that Steam would fail a signature check between the disk version of a DLL and a latent memory version." See, we were just thinking that that had something to do with it.
Newell begged pardon for the erroneous bans, adding "this was our mistake, and I apologize for any frustration or angst it may have caused you." An apology and a swift reversal of the ban aren't all that these users will receive -- Newell promised a giftable copy of Left 4 Dead 2 to all those affected, as well as one copy for themselves, provided they don't already have it.
[Thanks to everyone who sent this in!]